Expansion control of molds and the like



' plaster.

ream ng 2a, 1946 v osricr.

EXPANSION coN'raoL F MOLDS AND Xarifa L. Yellow Springs, Ohio, assignorto Antioch College, a corporation of Ohio, Morris Bean, Xarifa Bean,Algo D. Henderson, Arthur E. Morgan, and William Beatty, all of YellowSprings, Ohio No Drawing. Application October 15, 1943,

' Serial no. 500.425

Claims.

This invention relates generally to molds for use in metal casting andmore particularly to expansion control agents for use in plaster moldsin which the molten metal is to be poured in the casting process.

In the manufacture of non-ferrous metal castings, particularly aluminumand aluminum alloy castings in plaster of Paris'bonded molds, one of thedifficulties encountered with addition agents heretofore employed tocontrol the processing expansion of molds is that these addition agentshave sometimes caused an objectionable elilorescence or blooming of themold surface. Serious surface buckling may also result. To

overcome these difllculties requires expensive processing and constantinspection and retouching.

The problem has been encountered in the manufacture of aluminum alloycastings in molds according to the process of Bean Patent 2,220,703

' granted November 5, 1940. According to the Bean process, after theinitial set. the mold is placed in an autoclave, and subjected to steamunder pressure to convert calcium sulfate dihydrate to calcium sulfatehemi-hydrate and to leave free water distributed throughout the theautoclave and allowed to stand for a number of hours at room.temperature whereby the hemi-hydrate is rehydrated to di-hydrate. The

(01. 22-1ss) V The mold is thereafter removed from mold thereafteris'dried' to remove free water I fromthe plaster. The effect of this,treatment is to increase the permeability of the material so as topermit escape of gases generated in the mold during casting whileproviding a smooth skin at the surface of the gold giving betterdefinition to the surface of the casting.

Some have an adverse effect on the permeability.

drating steps of the Bean process and that these aluminates otherwisesuccessfully meet the requirements of a suitable control agent asoutlined.

The sodium or potassiumaluminate in powdered form is added to andthoroughly mixed with the dry plaster of Paris and the other componentsof the dry plaster mix. A small amount of control agentis all that isrequired. Very satisfactory results have been obtained with an additionof from.- .4% to 1% by weight of the sodium or potassium aluminate.

After the dry mixture including the "control agent has been thusprepared. water is added thereto to form a wet mix from which the moldis formed. After the wet plaster forming the mold has set, the'mold isprocessed with steam under pressure in an autoclave to convert thecalcium sulfate di-hydrate to calcium sulfate hemi-hydrate and to leavefree water distributed throughout the mold. The mold is removed from theautoclave and allowed to stand at room temperature for a number of hourswhereby the hemi-hydrate is rehydrated to di-hydrate; The mold is thendried to remove practically all of the remaining free water from theplaster.

In addition to being an effective expansion control agent, the sodium orpotassium aluminate is an inhibitor of "mold reaction. Mold reaction Theproblem of selecting suitable expansion the efiect of the control agenton other major properties and attributes of the mold .material, such assetting time, green strength, processing time, surface quality, drystrength, permeability and heat absorption rate. For example some Inaddition to controlling expansion control agents are powerfulaccelerators, causing the plaster to set almost instantly. Others arepowerful retarders or inhibitors.

causes a kind of porosity in the metal castings being produced, theamount of which canbe determined by X-ray analysis of the castings. f

It appears that the condition may be caused by gases formed by reactionbetween one or more of the components of the mold and one or more components of the molten metal or alloy being cast in the mold. Anillustrative example of an aluminum alloy which reacts with moldscontaining calcium sulfate is one composed of 5% silicon, 1.3% copper,0.5% magnesium, balance aluminum plus traces of impurities normallypresent in aluminum alloys. Sodium aluminate or potassium aluminate isan effective inhibitor 7 of mold reaction such as that encountered with,

the above described aluminum alloy.

Some agents will cause the green strength to be V but a fraction of whatit would ,be otherwise.

While the specific embodiment of the invention described herein dealswith expansion control agents for molds, it will be understood that inthe making of cores in which expansion is to be controlled.

Various changes and modifications of the embodiment of my inventiondescribed herein may he made without departing from the principle andspirit of the invention.

I claim: v 1. The method of controlling expansion in a plaster of Parismold or core which includes addme an alkali metal aluminate to the dryplaster mix. r

2. The method of controlling expansion in a plaster of Paris bonded moldor core which includes adding a small amount of an alkali metalaluminate to the dry mix.

3. A plaster of Paris mold containing a small amount of an alkali metalaluminate.

4. The method of forming a plaster of Paris mold which includes forminga desired mold shape from a mixture comprising plaster of Paris, a

small amountof an alkali metal aluminate and.

9,400,906 the principles of the invention have application water,setting the plaster, dehydrating the set plaster to convert calciumsulfate iii-hydrate to calcium sulfate hemi-hydrate and to leave freewater distributed throughout the mold, thereafter rehydrating the hem-hydrate to di-hydrate and then drying the mold to remove practicallyall of the remaining free water from the plaster;

5. A mold as in claim 3 in which the aluminate is sodium aluminate.

6. A mold as in claim 3 in which the aluminate is potassium aluminate.

7. The method as in claim 4 in which the aluminate is sodium aluminate.

8. The method. as in claim 4 in which the aluminate is potassiumaluminate.

9. A mold as in claim 3 in which about 4% to 1% by weight of sodiumaluminat is present in the mold.

10. The method as in claim 4 in which about .4% to 1% by weight ofsodium aluminate is employed.

XARIFA L. BEAN.

